RFR: 8236651: Simplify and update glass gtk backend

[Thiago Milczarek Sayao]

On Sun, 14 Jun 2020 11:55:45 GMT, Pankaj Bansal <pbansal at openjdk.org> wrote:

>> Following are results in Ubuntu 18.04 after fix for tab pane tests.
>> <img width="516" alt="Results_18 04"
>> src="https://user-images.githubusercontent.com/6153953/84587007-3abc2880-ae39-11ea-8b61-0cbb86e4d4b5.png">
>
> This is the result on OL 82 with latest commit
> <img width="596" alt="OL82"
> src="https://user-images.githubusercontent.com/6153953/84592508-d82c5200-ae63-11ea-87d3-7f55671ed302.png">

@pankaj-bansal Sorry for commiting again. Now the Tab Pane test works properly.

-------------

PR: https://git.openjdk.java.net/jfx/pull/77